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Visible Light Active Environmental Nanomaterials for the Oxidative Removal of Persistent Organic Pollutants from Water Environments

  • 이연준 (Youn-Jun Lee, 아주대학교 일반대학원)

초록/요약

Persistent organic pollutants (POPs) are a class of hazardous compounds characterized by their chemical stability, toxicity, and resistance to conventional degradation processes. These pollutants, including industrial chemicals, pesticides, and pharmaceutical residues, are widely detected in aquatic environments and pose significant risks to ecosystems and human health. Given their structural diversity and reactivity, the development of tailored treatment strategies is essential. Specifically, the degradation pathways of POPs can vary depending on their structure and characteristics. As such, it is critical to design treatment systems that consider these properties, selecting appropriate approaches—such as radical-driven photocatalysis or non-radical processes using oxidant combinations—for effective and selective removal. Visible light active environmental nanomaterials have emerged as promising candidates for the sustainable degradation of POPs. These materials can harness solar light to activate photocatalytic reactions, offering an energy-efficient alternative to UV-based systems. Their tunable properties allow for the rational design of catalysts that generate specific reactive species. Furthermore, the combination of photocatalysts with oxidants enables expanded reactivity beyond conventional pathways. This study explores the development and application of advanced visible light-driven catalysts, with and without oxidants, for the selective degradation or polymerization of POPs, emphasizing the importance of matching and optimizing catalyst systems to pollutant characteristics to maximize treatment performance. In Chapter 3, sulfate-doped silver phosphate (SO4-Ag3PO4) was prepared using a simple precipitation method, and its visible light photocatalytic activity against seven neonicotinoid insecticides currently available on the market was evaluated. The characteristics of the photocatalysts were analyzed using diffuse reflectance-UV/visible spectrophotometer measurements and electrochemical impedance spectroscopy analysis. Photocatalytic degradation of all tested insecticides under visible light irradiation was significantly enhanced by SO4 doping, which decreased band gap energy and charge transfer resistance. The apparent first- order rate constant (kapp) with SO4-Ag3PO4 varied depending on the insecticides (0.003-0.432/min), and was at least 5.4-fold faster than that with pristine Ag3PO4, in the order of thiacloprid > nitenpyram > imidacloprid > clothianidin > acetamiprid > thiamethoxam > dinotefuran. Even after four reuse cycles, SO4-Ag3PO4 maintained over 75% of its initial photocatalytic efficiency. Reactive species trapping experiments indicated that photoinduced electron holes (h+) were the most important oxidant for ICP degradation. In Chapter 4, a novel strategy for fabrication of N-vacancy-rich g-C3N4 (NvrCN) via post-solvothermal treatment of Mg-doped g-C3N4 was explored. The addition of the Mg precursor during the polycondensation of urea created abundant amine sites in the g-C3N4 framework, which facilitates formation of N vacancies during post-solvothermal treatment. Elemental analysis and electron paramagnetic resonance spectra confirmed a higher abundance of N vacancies in the resultant NvrCN. Further optical and electronic analyses revealed the beneficial role of N vacancies in light-harvesting capacity, electron-hole separation, and charge transfer. N vacancies also provide specific reaction centers for O2 molecules, promoting oxygen reduction reaction. Therefore, •OH generation increased via enhanced formation of H2O2 under visible light irradiation, and NvrCN photocatalytically degraded oxytetracycline 4-fold faster with degradation rate constant of 1.85 × 10-2 min-1 (light intensity = 1.03 mW/cm2, catalyst concentration = 0.6 g/L, oxytetracycline concentration = 20 mg/L) than pristine g-C3N4. Overall, this chapter provides a facile method for synthesizing N-vacancy-rich g-C3N4 and elucidates the role of the defect structure in enhancing the photocatalytic activity of g-C3N4. In Chapter 5, visible light was utilized to shift the reaction pathway from phenolic pollutants degradation (which often results in incomplete mineralization) to polymerization over a g–C₃N₄ catalyst deposited on a Cu₂O nanowire. In peroxymonosulfate (PMS) activation, visible light illumination facilitates electron transfer from Cu₂O to g–C₃N₄, shifting the dominant reactive species on the Cu₂O surface from radical species to catalyst–PMS* complex. Experimental results and theoretical calculations verified the increased adsorption of PMS and the reduced energy barrier for phenol (PhOH) polymerization under visible light illumination. Compared to dark control conditions, visible light-assisted Fenton-like process resulted in a 9-fold faster PhOH oxidation (0.029 min⁻¹), significantly higher PMS utilization efficiency for total organic carbon removal, and enhanced robustness in real water matrices. This chapter provides fundamental insights into the regulation of reaction pathway over light-responsive semiconductor catalysts and highlights the potential for efficient treatment of phenolic pollutants via oxidative polymerization. This study advances reactive species analysis, removal pathway identification, and practical application of visible light driven treatment technologies for removal of POPs. Ultimately, the findings contribute to demonstrate the potential of using visible light active nanomaterials to develop sustainable and efficient strategies for removing diverse POPs for wastewater remediation. Keywords: Advanced oxidation process, visible light, environmental nanomaterials, persistent organic pollutants, oxidation mechanism, reactive species

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목차

Chapter 1. Introduction １
1.1. General overview １
1.2. Objective of the research ４
Chapter 2. Literature review ６
2.1. Visible light active heterogeneous catalysts for water treatment ６
2.1.1. Principles of visible light photocatalysis ６
2.1.2. Factors affecting the efficiency of visible light active catalysts ８
2.1.3. Catalyst modification strategies １１
2.1.4. Visible light active catalysts combined with oxidants １３
2.2. Oxidation mechanism of organic pollutants １６
2.2.1. Oxidative degradation of organic pollutants １６
2.2.2. Oxidative polymerization of organic pollutants １９
Chapter 3. Photocatalytic degradation of neonicotinoid insecticides using sulfate-doped Ag3PO4 with enhanced visible light activity ２２
3.1. Introduction ２２
3.2. Materials and Methods ２４
3.2.1. Chemicals ２４
3.2.2. Catalyst Preparation ２５
3.2.3. Characterizations ２６
3.2.4. Experimental procedures ２６
3.2.5. Analytical methods ３１
3.3. Results and Discussion ３３
3.3.1. Characterization of Ag3PO4 and SO4-Ag3PO4 ３３
3.3.2. SO4-Ag3PO4 significantly outperformed Ag3PO4 under visible light irradiation ３７
3.3.3. Application to neonicotinoid insecticides degradation ４３
3.3.4. Stability and applicability of SO4-Ag3PO4 ４８
3.3.5. Degradation mechanism mainly involves direct ICP oxidation by photoinduced holes ５２
3.4. Summary ５９
Chapter 4. Facile synthesis of N vacancy g-C3N4 using Mg- induced defect on the amine groups for enhanced photocatalytic •OH generation ６０
4.1. Introduction ６０
4.2. Materials and Methods ６２
4.2.1. Chemicals ６２
4.2.2. Preparation of Mg dispersed g-C3N4 (MgCN) ６３
4.2.3. Preparation of N vacancy-rich g-C3N4 (NvrCN) ６３
4.2.4. Catalyst Characterization ６４
4.2.5. Photocatalytic Experiments ６６
4.2.6. Photoelectrochemical Analysis ６７
4.3. Results and Discussion ６９
4.3.1. N defects were effectively introduced by post-solvothermal treatment of Mg- dispersed, NHx-abundant g-C3N4 ６９
4.3.2. Optical and electronic properties of g-C3N4 were enhanced by N vacancy introduction. ７７
4.3.3. The introduced N defects enhanced photocatalytic •OH generation and OTC degradation ８３
4.4. Summary ９３
Chapter 5. Visible light-driven selective redirection of phenolic carbon in heterogeneous Fenton-like reaction. ９４
5.1. Introduction ９４
5.2. Materials and Methods ９７
5.2.1. Chemicals ９７
5.2.2. Synthesis of Catalyst ９７
5.2.3. Visible Light-assisted Fenton-like Experiments ９８
5.2.4. Characterizations ９９
5.2.5. Analytical Methods １００
5.2.6. Photoelectrochemical analysis １０３
5.2.7. Computational methods １０６
5.2.8. Estimation of electrical energy per log order (EE/O) １０６
5.3. Results and Discussion １０８
5.3.1. Heterojunction with Cu2O nanowire improves charge transfer of g–C3N4 under visible light illumination １０８
5.3.2. Visible light enhanced heterogeneous Fenton-like activity and increased PMS utilization efficiency for TOC removal and polymerization of phenolic pollutants １１４
5.3.3. Transformation of dissolved phenolic pollutant into recoverable solid product under visible light illumination １２４
5.3.4. Enhanced selective polymerization of the phenolic compound by altered reactive species under light illumination １２７
5.3.5. Decreased energy barrier of PhOH polymerization pathway and increased resistance to background substances under visible light illumination １３７
5.4. Summary １４７
Chapter 6. Conclusion and future work １４８
6.1. Conclusion １４８
6.2. Future work １５０
References １５３

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